Index: include/llvm/Analysis/BasicAliasAnalysis.h
===================================================================
--- include/llvm/Analysis/BasicAliasAnalysis.h
+++ include/llvm/Analysis/BasicAliasAnalysis.h
@@ -185,14 +185,17 @@
 
   AliasResult aliasPHI(const PHINode *PN, uint64_t PNSize,
                        const AAMDNodes &PNAAInfo, const Value *V2,
-                       uint64_t V2Size, const AAMDNodes &V2AAInfo);
+                       uint64_t V2Size, const AAMDNodes &V2AAInfo,
+                       const Value *UnderV2);
 
   AliasResult aliasSelect(const SelectInst *SI, uint64_t SISize,
                           const AAMDNodes &SIAAInfo, const Value *V2,
-                          uint64_t V2Size, const AAMDNodes &V2AAInfo);
+                          uint64_t V2Size, const AAMDNodes &V2AAInfo,
+                          const Value *UnderV2);
 
   AliasResult aliasCheck(const Value *V1, uint64_t V1Size, AAMDNodes V1AATag,
-                         const Value *V2, uint64_t V2Size, AAMDNodes V2AATag);
+                         const Value *V2, uint64_t V2Size, AAMDNodes V2AATag,
+                         const Value *O1 = nullptr, const Value *O2 = nullptr);
 };
 
 /// Analysis pass providing a never-invalidated alias analysis result.
Index: lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- lib/Analysis/BasicAliasAnalysis.cpp
+++ lib/Analysis/BasicAliasAnalysis.cpp
@@ -1102,7 +1102,8 @@
       return MayAlias;
 
     AliasResult R = aliasCheck(UnderlyingV1, MemoryLocation::UnknownSize,
-                               AAMDNodes(), V2, V2Size, V2AAInfo);
+                               AAMDNodes(), V2, V2Size, V2AAInfo,
+                               nullptr, UnderlyingV2);
     if (R != MustAlias)
       // If V2 may alias GEP base pointer, conservatively returns MayAlias.
       // If V2 is known not to alias GEP base pointer, then the two values
@@ -1239,7 +1240,8 @@
 AliasResult BasicAAResult::aliasSelect(const SelectInst *SI, uint64_t SISize,
                                        const AAMDNodes &SIAAInfo,
                                        const Value *V2, uint64_t V2Size,
-                                       const AAMDNodes &V2AAInfo) {
+                                       const AAMDNodes &V2AAInfo,
+                                       const Value *UnderV2) {
   // If the values are Selects with the same condition, we can do a more precise
   // check: just check for aliases between the values on corresponding arms.
   if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
@@ -1257,12 +1259,14 @@
   // If both arms of the Select node NoAlias or MustAlias V2, then returns
   // NoAlias / MustAlias. Otherwise, returns MayAlias.
   AliasResult Alias =
-      aliasCheck(V2, V2Size, V2AAInfo, SI->getTrueValue(), SISize, SIAAInfo);
+      aliasCheck(V2, V2Size, V2AAInfo, SI->getTrueValue(),
+                 SISize, SIAAInfo, UnderV2);
   if (Alias == MayAlias)
     return MayAlias;
 
   AliasResult ThisAlias =
-      aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(), SISize, SIAAInfo);
+      aliasCheck(V2, V2Size, V2AAInfo, SI->getFalseValue(), SISize, SIAAInfo,
+                 UnderV2);
   return MergeAliasResults(ThisAlias, Alias);
 }
 
@@ -1270,8 +1274,8 @@
 /// another.
 AliasResult BasicAAResult::aliasPHI(const PHINode *PN, uint64_t PNSize,
                                     const AAMDNodes &PNAAInfo, const Value *V2,
-                                    uint64_t V2Size,
-                                    const AAMDNodes &V2AAInfo) {
+                                    uint64_t V2Size, const AAMDNodes &V2AAInfo,
+                                    const Value *UnderV2) {
   // Track phi nodes we have visited. We use this information when we determine
   // value equivalence.
   VisitedPhiBBs.insert(PN->getParent());
@@ -1350,7 +1354,8 @@
     PNSize = MemoryLocation::UnknownSize;
 
   AliasResult Alias =
-      aliasCheck(V2, V2Size, V2AAInfo, V1Srcs[0], PNSize, PNAAInfo);
+      aliasCheck(V2, V2Size, V2AAInfo, V1Srcs[0],
+                 PNSize, PNAAInfo, UnderV2);
 
   // Early exit if the check of the first PHI source against V2 is MayAlias.
   // Other results are not possible.
@@ -1363,7 +1368,7 @@
     Value *V = V1Srcs[i];
 
     AliasResult ThisAlias =
-        aliasCheck(V2, V2Size, V2AAInfo, V, PNSize, PNAAInfo);
+        aliasCheck(V2, V2Size, V2AAInfo, V, PNSize, PNAAInfo, UnderV2);
     Alias = MergeAliasResults(ThisAlias, Alias);
     if (Alias == MayAlias)
       break;
@@ -1376,7 +1381,8 @@
 /// array references.
 AliasResult BasicAAResult::aliasCheck(const Value *V1, uint64_t V1Size,
                                       AAMDNodes V1AAInfo, const Value *V2,
-                                      uint64_t V2Size, AAMDNodes V2AAInfo) {
+                                      uint64_t V2Size, AAMDNodes V2AAInfo, 
+                                      const Value *O1, const Value *O2) {
   // If either of the memory references is empty, it doesn't matter what the
   // pointer values are.
   if (V1Size == 0 || V2Size == 0)
@@ -1404,8 +1410,11 @@
     return NoAlias; // Scalars cannot alias each other
 
   // Figure out what objects these things are pointing to if we can.
-  const Value *O1 = GetUnderlyingObject(V1, DL, MaxLookupSearchDepth);
-  const Value *O2 = GetUnderlyingObject(V2, DL, MaxLookupSearchDepth);
+  if (O1 == nullptr)
+    O1 = GetUnderlyingObject(V1, DL, MaxLookupSearchDepth);
+
+  if (O2 == nullptr)
+    O2 = GetUnderlyingObject(V2, DL, MaxLookupSearchDepth);
 
   // Null values in the default address space don't point to any object, so they
   // don't alias any other pointer.
@@ -1488,23 +1497,26 @@
 
   if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
     std::swap(V1, V2);
+    std::swap(O1, O2);
     std::swap(V1Size, V2Size);
     std::swap(V1AAInfo, V2AAInfo);
   }
   if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
-    AliasResult Result = aliasPHI(PN, V1Size, V1AAInfo, V2, V2Size, V2AAInfo);
+    AliasResult Result = aliasPHI(PN, V1Size, V1AAInfo,
+                                  V2, V2Size, V2AAInfo, O2);
     if (Result != MayAlias)
       return AliasCache[Locs] = Result;
   }
 
   if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
     std::swap(V1, V2);
+    std::swap(O1, O2);
     std::swap(V1Size, V2Size);
     std::swap(V1AAInfo, V2AAInfo);
   }
   if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
     AliasResult Result =
-        aliasSelect(S1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo);
+        aliasSelect(S1, V1Size, V1AAInfo, V2, V2Size, V2AAInfo, O2);
     if (Result != MayAlias)
       return AliasCache[Locs] = Result;
   }